1. Field of the Invention
This invention relates to a photoelectric conversion device, and more particularly, to a photoelectric conversion device used as an input unit for a facsimile, an image reader, a digital copying machine, an electronic blackboard or the like.
2. Description of the Related Art
Recently, long line sensors having optical systems with one-to-one magnification ratio have been developed as photoelectric conversion devices for the purpose of providing small and high-performance facsimiles, image readers, and the like.
Furthermore, for the purpose of providing small and inexpensive devices, photoelectric conversion devices have also been proposed in which a sensor directly detects light reflected from an original via a transparent spacer made of glass or the like without using a fiber lens array with one-to-one magnification ratio.
FIGS. 1(A) and 1(B) schematically show an example of a photoelectric conversion device. FIG. 1(A) is a schematic cross-sectional view of the photoelectric conversion device, as seen from the main scanning direction of a photoelectric conversion element array. FIG. 1(B) is a schematic cross-sectional view of the device, as seen from the subscanning direction of the photoelectric conversion element array.
In FIGS. 1(A) and 1(B), a transparent sensor substrate 1 includes photoelectric conversion elements (not shown) formed on a transparent substrate, made of glass or the like, by a semiconductor process or the like, and an illuminating window (not shown). A transparent mounting substrate 2 includes an interconnection wiring member 4 formed on a transparent substrate, made of glass or the like, by a thick-film printing method, a photolithographic method or the like. The interconnection member 4 electrically connects the transparent sensor substrate 1 to a driving circuit unit (not shown) provided on the transparent mounting substrate 2. The transparent sensor substrate 1 is bonded on the transparent mounting substrate 2 by an adhesive layer 5. A light source 3 for emitting light for illuminating an original P comprises an LED array consisting of a plurality of LED chips 6 arranged in the form of an array and a light-condensing member 33 provided thereon.
The reading position of the original P, the arranged position of the illuminating window in the transparent sensor substrate 1 and the optical axis of the light source 3 in the direction of the array are set to positions which exist within a vertical plane descending from the reading position of the original P, as shown by arrow L.
Illuminating light L from the light source 3 is projected onto the original P after passing through the transparent mounting substrate 2 and the illuminating window within the transparent sensor substrate 1. Information light reflected from the original P is incident upon the photoelectric conversion elements on the transparent sensor substrate 1, and is subjected to photoelectric conversion by the photoelectric conversion elements. The converted signal is output to the outside as an image signal.
FIGS. 1(C) and 1(D) show a modified example of the photoelectric conversion device shown in FIGS. 1(A) and 1(B). The modified device differs from the device shown in FIGS. 1(A) and 1(B) only in that there is no light-condensing member 33 provided in the light source 3.
However, in attempts to provide a further inexpensive and smaller device, it has become clear that the above-described photoelectric conversion devices have the following technical problems.
As a means for providing an inexpensive photoelectric conversion device, it is possible to reduce the number of the LED chips and thus to reduce the cost of the light source.
When the number of the LED chips are reduced, illuminance on the surface of the original is reduced, causing uneven illuminance. This substantially deteriorates the quality of an image, especially when a white original is read.
In order to reduce the uneven illuminance, it is possible to increase the distance between the LED array and the surface of the original. This approach, however, is by no means preferable, since the illuminance substantially decreases and the photoelectric conversion device becomes large.
On the other hand, when an original is illuminated by the LED array, much light beams nearly perpendicular to the surface of the original are incident upon the surface of the original situated just above the LED chips, but much light beams inclined relative to the surface of the original are incident upon the surface of the original situated at positions above positions between the LED chips. That is, when an original is illuminated by a so-called pseuodo-linear light source in which a plurality of light-emitting sources are arranged, a directional property which the light beams of the illuminating light have causes uneven directional property of the light beam within the area of the surface of the original. If the illuminating light has such a directional unevenness, the quality of an image is substantially deteriorated when a black original is read.
In order to prevent the directional unevenness in the illuminating light, it is possible to use a linear light source, such as a fluoroscent lamp. The use of such a light source, however, finds difficulty in providing an inexpensive and small device, since such a light source generally has large external dimensions and is apt to be broken, and the application of an AC voltage, of high voltage and high frequency in some cases, converted from a DC voltage is needed in order to light the light source.
The above-described technical problems will now be explained in detail.
FIG. 2 shows image signal outputs from the photoelectric conversion device shown in FIG. 1 when a white original and a black original are read.
According to FIG. 2, it can be understood that an image signal output (a) when a white original is read is nearly uniform over the width of the original, while an image signal output (b) when a black original is read has variations with a large period which corresponds to the arranged pitch of the LED chips. Since the magnitude of the variations in the image signal output when the black original is read is as small as a few % compared with the magnitude of the image signal output when the white original is read, there is practically no problem when the photoelectric conversion device shown in FIG. 1 is used for reading two, i.e., black and white, values.
When the device is used for reading multiple gradations, however, although the magnitude of variations in an image output when a black original is read is small, a noise corresponding to the arranged pitch of the LED chips appears as stripes in a reproduced output image corresponding to the black original in some setting of slice levels for providing the color density of the image of the original with multiple values. This deteriorates the picture quality.
This problem, which has been found by the inventors of the present invention, is an inherent technical problem which arises when a so-called pseudo-linear light source, in which a plurality of light-emitting sources are arranged in order to provide uniform illuminance on the surface of an original, is combined with a so-called lensless photoelectric conversion device which directly detects light reflected from the original without using a lens array. In other words, the so-called lensless photoelectric conversion device has light source illuminating the original through the transparent substrate including the photoelectric conversion elements. Accordingly, this problem does not arise when a linear light source, such as a fluorescent lamp, is used.
Before explaining embodiments according to the present invention, the above-described problem will be explained by reference to the accompanying drawings.
Even in a pseudo-linear light source such as an LED array or the like, the amounts of light beams illuminating the surface of an original are nearly identical at a position just above an LED chip and at a position above a position between adjacent LED chips.
When a white original, for example, is illuminated by an LED array, illuminating light beams L are reflected from an original P, and reflected light beams L' are incident upon photoelectric cnversion elements 8, as shown in FIGS. 3(A) and 3(B). There is also shown a transparent spacer 9.
When the white original is illuminated, the reflected light L' has a diffused-light component which randomly diffuses with not at all depending on the incident angle of a light beam upon the white original and a specular-reflection-light component which performs specular reflection depending on the incident angle of a light beam.
FIG. 3(A) shows the behavior of reflected light incident upon photoelectric conversion elements situated just above LED chips.
As shown in FIG. 3(A), very few specular-reflection-light component is included within the compoment of the reflected light incident upon the photoelectric conversion elements situated just above the LED chips. Since the illuminating light L passes through the illuminating window in the transparent sensor substrate 1 and illuminates the original P with an angle almost perpendicular to the original P, the specular-reflection-light component L' passes again through the illuminating window and goes to the outside.
FIG. 3(B) shows the behavior of reflected light incident upon photoelectric conversion elements situated at positions above positions between adjacent LED chips.
As shown in FIG. 3(B), both the diffused-light component and the specular-reflection-light component are incident upon the photoelectric conversion elements situated at positions above positions between the LED chips.
Accordingly, if a white original is illuminated by a pseudo-linear light source, the amount of light beams incident upon photoelectric conversion elements situated just above LED chips becomes smaller by an amount of the specular-reflection-light component. However, since the diffused-light component is much larger than the specular-reflection-light component in the component of the light reflected from the surface of the white original, variations in the amount of light beams incident upon the photoelectric conversion elements practically cause no problem.
On the other hand, when a black original is illumunated by a pseudo-linear light source, since illuminating light beams are absorbed on the surface of the original and are hardly diffused, as shown in FIGS. 4(A) and 4(B), the specular-reflection-light component is emphasized.
As shown in FIG. 4(A), since the illuminating light is absorbed by the black original, the diffused light and the specular-reflection-light component L' reflected are hardly incident upon photoelectric conversion elements situated just above LED chips.
To the contrary, as shown in FIG. 4(B), the specular-reflection-light component is incident upon photoelectric conversion elements situated at positions above positions between the LED chips. This is because a black original is generally prepared by printing black ink on white paper, and it is difficult to prevent the generation of the specular-reflection-light component from the surface of the black ink. Thus, when a black original is read, variations in the amount of light beams incident upon photoelectric conversion elements are further increased.